The present invention generally relates to the field of materials deposition for a component fabrication, restoration, and surface improvement and more particularly, is directed to a process and apparatus for coating or cladding a layer or layers of metal and non-metal materials onto substrates of dissimilar materials.
There are variety of methods known in the prior art for coating or cladding a layer of dissimilar material onto a substrate. Some examples of the known methods that produce enhanced bonding between the coating and the substrate as follows:    1. Thermal Spray processes,    2. Conventional Arc Welding,    3. Laser and Electron Beam metal deposition,    4. Large Scale Explosion Welding and Micro-explosion Welding.
The terms “coating” and “cladding” although sometimes used interchangeably, usually refer to different application processes. Cladding refers to a coating layer production process by bonding a plate or film to a substrate, while coating refers to a coating layer production process by bonding of materials in the form other than a plate or a film for example, liquids, powders or wires.
Thermal Spray Coating processes such as plasma spray or high-velocity oxy-fuel (HVOF) methods produce a relatively weak mechanical bond with a substrate and work well in applications where a component is subjected primarily to compression loads. In more demanding applications where a component is subjected to torsion, shear, or bending stresses, the coating needs to have a strong metallurgical bond with a substrate.
Metallurgical bond strength welding can be produced with such welding methods as arc welding, plasma, laser powder deposition, and Electron Beam wire or powder deposition. However, these welding methods apply an excessive heat and a melt-pool at the substrate for a time sufficient to result in melting and intermixing of elements of the alloys being joined. Such liquid phase intermixing and associated chemical reactions have been observed to result in the creation of brittle intermetallic compounds at the joint interface leading to coating cracking and flaking.
If the coating and substrate are metallurgically compatible or “weldable”, then conventional welding techniques may be used. This process is sometimes referred to as fusion welding or hardfacing. Fusion hardfacing cannot be used in cases where the substrate cannot be heated to the liquefaction state or the materials are not weldable. For example, coating of aluminum with high temperature and strength metals, such as titanium and nickel alloys, can often be bonded by an explosion welding process. Explosion welding belongs to a solid-state welding methods allowing for a strong metallurgical bond between materials of dissimilar physical characteristics and without substantial liquefaction of the substrate.
The explosion welding method is based on high velocity oblique collision of two metal plates. The cladding plate (the “flyer”) is covered with an explosive material and placed at some distance from the substrate. The explosive material is detonated and it propagates though the entire surface of the flyer plate. The flyer plate collides with the substrate at velocities in the range of 400-3000 m/sec. The high collision pressure and heat created at the collision line allows hydrodynamic flow to occur creating a strong wavy interface between dissimilar metals. The shortcoming of this process is in its high cost and its safety issues due to handling and controlling explosives. Another limitation of the process is that it is primarily suitable for cladding of flat surfaces.
Other methods similar to explosion welding are using laser energy to evaporate the top layer of a foil or a separate sacrificial layer to realize the recoil forces, generated by the high velocity of evaporated gases, to drive the coating foils toward the target substrate.
Drew et al T988007 (1979) discloses a laser vapor deposition technique wherein a CW laser beam is directed through a transparent substrate onto a reservoir of metal on the opposite side of and spaced from the substrate. The laser beam heats and vaporizes the metal of the reservoir, which is then redeposited on the opposing surface of the substrate.
Mayer et al, “Pulsed Laser Microfabrication” U.S. Pat. No. 4,752,455, proposes a system and method of pulsed-laser microfabrication wherein a first substrate of transparent material, such as glass, has a conductive film and then a target material both positioned on a surface of the transparent substrate. The target material is placed immediately adjacent to the target substrate surface. Pulsed laser energy is directed through the transparent substrate onto the conductive film at a sufficient intensity and for a sufficient duration to rapidly vaporize the metal film. The target materials are driven by film vaporization energy and by the reaction thereof against the glass substrate onto the opposing or object surface of a second substrate.
This process resembles the large scale explosion welding but on a micro-level, often referred to as a micro-explosion welding, and is limited to cladding of very thin films and primarily on flat surfaces.
Frish et al, “Method for Bonding Using Laser Induced Heat and Pressure” U.S. Pat. No. 4,684,781, proposes bonding metal foils to substrates of dissimilar metal by placing the foil in contact with the substrate and then irradiating the foil surface remote from the substrate with a laser pulse so as to ablate a portion of the foil surface.
According to the invention, a foil layer of a material to be applied as a coating to a substrate is laid over and in contact with the substrate surface which is to be coated. A laser beam, typically pulsed, and of a very high instantaneous intensity is applied to the exposed foil layer.
The intensity of the radiation as focused onto the foil at a small spot is extremely high, substantially greater than that used in prior applications such as laser welding. The extremely high laser intensity produces an instantaneous vaporization of a small portion of the foil surface. The reaction to the vapor pressure or “recoil force” generated by the foil surface vaporization is a pressure or shock wave transmitted in the opposite direction and through the thin foil layer in the direction of the interface between the foil and substrate. Laser power densities, typically usable in the above invention, range from hundreds of megawatts per square centimeter to hundreds of thousands of megawatts per square centimeter with pulse durations measured in a few hundredths of a microsecond up to over twenty microseconds.
Thermal and pressure waves are generated in the foil and travel through the foil thickness at differing velocities. If the thermal wave reaches the foil/substrate interface during irradiation, both materials will melt and thereafter mix under the influence of the laser-induced pressure gradients. Thus, the disclosed “laser stamping” technique makes use of both heat and pressure supplied to the foil by the high intensity laser pulse.
The major shortcomings of this method are as follows:                1. The foil is pre-placed onto a substrate with sufficiently large gaps present between the foil and the substrate due to the initial imperfection in flatness of a foil and the substrate as well as the additional irregular gaps created during the laser-irradiation of the foil. These gaps prevent the pressure and thermal waves from reaching the substrate and thus prevent the production of a reliable uniform bond of the foil to the substrate.        2. Because of the above irregular gaps between the pre-placed foil and the substrate, an extremely high power densities and very thin foils (2 to 100 micrometers) are required in order to create sufficient momentum to propel the foil toward the substrate against the gap resistance.        3. In addition, these extremely high power intensities of the laser beam delivered to a very thin foil, having a plurality of gaps of different distances, would tend to perforate the foil in many places where there is a gap between a foil and the substrate, producing unacceptable quality coatings.        
Rabinovich “Rapid Prototyping System” U.S. Pat. No. 5,578,227, which is fully incorporated herein by reference, describes a part making method and apparatus with laser assisted fusion of a rectangular wire to a substrate. In this method the feedstock fusion is performed with a laser beam of the size and power density sufficient for reliable spot or continuous fusion welding of the feedstock to the previous layer while keeping the feedstock cross-section in substantially original shape and producing fusion with a limited local heat input into the part. This limited heat input is achieved through the wire surface-to-surface contact with the substrate, which provides good thermal conductivity to heat flow. This effective heat conduction eliminates the need in the creation of a molten-pool in the substrate prior to metal deposition, as it is the case in other energy beam metal deposition processes, which utilize metal powders or round wires. Although the advantages of this melt-pool free, ultra-low heat input process have been instrumental for repairs of many complex and thin wall structures and many dissimilar materials, the true solid-state welding capabilities required for strong metallurgical bonding of many dissimilar materials was not anticipated at that time.
While practicing the method of U.S. Pat. No. 5,578,227, under a trademark of Precision Metal Deposition (PMD™), the author discovered new and unexpected qualities of this method, which with a number of improvements, allow bonding of extremely dissimilar metals to be performed in a solid-state mode of welding.